While most cellular phone systems implemented today use land-based communication networks, personal satellite communication systems have been developed and continue to evolve to allow communication between any two places on earth much easier and efficient. Typically, international telecommunication satellites operate from a geo-stationary orbit (GEO) that is approximately 36,000 kilometers above earth. From this point, the satellite appears to remain fixed above a single spot on the Earth. Alternatively, telecommunication satellites have also been implemented in orbits closer to Earth that are referred to as low Earth orbits (LEOs) and medium Earth orbits (MEOs). Satellites in LEOs orbit at altitudes between 780 kilometers and 1390 kilometers. Satellites in MEOs orbit at an altitude of approximately 10,000 kilometers.
FIG. 1 illustrates a typical satellite environment, where the satellite may be orbiting at either a geostationary orbit, a low Earth orbit, or a medium Earth orbit. However, for simplicity in this explanation, it will be assumed that satellite 102 of FIG. 1 is a GEO-based satellite. GEO mobile satellite systems are able to determine where mobile subscribers are located and where traffic is routed based on information stored in a home location register (HLR), such as HLR 114. Additionally, GEO mobile satellite systems may also determine the location of subscribers based on a visiting location register (VLR), such as that included, but not illustrated in detail, in Gateway A 110 and Gateway B 104. Through the selective use of the HLR and VLR, locations of mobile telephone subscribers may be determined to within a small service area. In addition to an HLR that stores subscriber service profile and location information, a typical GEO mobile satellite network includes a mobile switching center (MSC) for performing call processing, connection control, and traffic routing that take place between a public switched telephone network (PSTN) 108 and the GEO mobile satellite network. In FIG. 1, both Gateway A 110 and Gateway B 104 include MSCs (not illustrated in detail). Additionally, Gateway A 110 and Gateway B 104 comprise radio and satellite interface units (SIU) for communication and transmission to and from satellite 102. During operation, satellite 102 acts as a radio system with multiple beams. The beams output by satellite 102 are different for gateways (104, 110) and mobile subscriber terminals (106). In general, the beams used by satellite 102 to communicate with mobile subscriber terminal 106 are narrow spot beams with high power density. In contrast, the beams output by satellite 102 to communicate with gateways (104, 110) usually allow for broader coverage and have a lower power density. It should be noted that satellite 102 may support one or multiple gateway beams, and, in general, many or multiple mobile subscriber terminal beams. Operation of each of the components of Gateway A 110 and Gateway B 104 are well-known to those with skill in the data processing art and, therefore, will not be described in greater detail herein.
FIG. 1 also illustrates a mobile subscriber terminal 106 where telephone calls are delivered or originated, and a network control center 112 where communications between satellite 102 and gateways 10 and 104, as well as frequency allocations, are managed and monitored. Furthermore, the satellite communication network 100 of FIG. 1 contemplates the use of a short message service (SMS) and a voice mail service (VMS) that is provided via data processor 118.
During operation of the satellite communication system, the uplink/downlink between satellite 102 and Gateways 104 and 110 operates at a high bandwidth, usually in the 100 MHz range. Furthermore, each Gateway (104 and 110) is assigned a set of frequencies which it transmits and receives traffic to and from satellite 102. In GEO-based communication networks, the satellite selectively assigns an incoming slot to a selected downlink beam. By using such techniques, the satellite may act as a switch or cross-connect between the two Gateways.
In connecting the two gateways, the analogy between the satellite cellular structure and the cellular mobile telephone system becomes more apparent. In the cellular mobile system, cellular sites are fixed and users are mobile. As the user of a mobile telephone system travels from one cell site to another, the user's telephone call is handed off from one cellular switching unit to another. Conversely, users of the LEO satellite cellular system are relatively fixed at any given time, while the satellites, which are the cells, are in continuous movement. With the hand-held or vehicle mounted mobile telephone, connection to one of the satellite switches is made directly from the hand-held mobile unit (MS 106) or a remotely fixed telephone (not illustrated in detail herein) to one of the nearest satellite moving about the earth. As the satellite that originally serves a particular user leaves a cell of that switch, the user's telephone call is "handed off" to an appropriate adjacent cell. Adjacent cells may be cells within one satellite or cells of other satellites located either in a particular orbiting plane or an adjacent orbiting plane. Furthermore, while users may "roam," this roaming speed is relatively small compared to the travelling speed of the satellite switches.
In another similarity to the cellular mobile telephone system, the satellite cellular communication system provides spectral efficiency. This means that the same frequency may be simultaneously used by different satellites, wherein spectral efficiency is provided by the special diversity between the satellite and users. Additionally, the users may be located anywhere on a land mass, on the water or in the air at an altitude less than that of the low-earth orbiting satellites. For example, a person on one land mass could call a person on another land mass, a person on a boat or a person in an aircraft. For additional information about a satellite cellular telephone and data communication system, refer to U.S. Pat. No. 5,604,920 by Bertiger, et al., which is hereby incorporated by reference herein.
During operation of a satellite communication system 100 of FIG. 1, it should be noted that service provided by the satellite may be interrupted for a myriad of reasons. Refer now to FIG. 2. As illustrated in FIG. 2, one predictable outage is the result of the sun and the electromagnetic field generated by the sun's rays during particular points in time. During service disruptions that occur as a result of a solar outage, otherwise known as a sun transit, the sun, satellite and gateway line up in a format that allows the electromagnetic field generated by the sun's rays to overwhelm the downlink between the satellite and the gateways for a brief period of time. For a GEO satellite system, this disruption typically occurs for two to four minutes once a week. Sun transit is a well-known phenomenon and is very predictable.
However, during the period of time in which a solar outage occurs, communication for mobile system users is interrupted, thus significantly degrading the quality of service provided to the mobile user. Additionally, such interruptions may be extremely detrimental in time sensitive communications that may be in process. Furthermore, given the fact that an entire gateway is affected by this solar transit problem, a whole group of subscribers is impacted by such a communication interruption. Thus, such interruptions, although predictable, adversely affect large numbers of mobile subscribers.
It should be noted that outages other than sun transit outages are also predictable in GEO-based satellite networks. Such additional outages may be due to maintenance or repair, among others.
Therefore, a need exists for a mechanism within a mobile satellite communication network that compensates for such predictable outages and allows users to continue their telephone calls, even when such predictable outages occur for some of the many satellite-to-surface communication links.